Head or helmet mounted display systems find utility in many areas, such as training, machine control or entertainment. In recent years, such devices have found particular utility in military applications where information must be supplied to vehicle operators and weapons controllers. Aircraft operators find such devices useful to furnish information relating to aircraft operation, radar presentations, maps, weapons aiming and other things, such as symbols, that would be useful or desirable in their mission.
In prior devices, images from a variety of sources, including cathode ray tubes (CRTs) have been used to project images which are superimposed on a surface in the user's line of sight. Helmet mounted systems for displaying images are well known in the art. Several systems have been described in which the optics may be head mounted with a cathode ray tube and heavy collimating lenses being placed in a cylinder and mounted on either the right or left side of a pilot's helmet. This weight placement is a considerable disadvantage to the pilot.
U.S. Pat. No. Reg. 28,847 to Vizenor, presently assigned to Honeywell Inc., described a display where the helmet was provided with a paraboloidal visor with a partial reflective coating on the inner surface as the primary optical element in transmitting the image from the generating device to the eye of the observer. A light source image or a virtual image is positioned at the focal point of the inside surface of the visor.
A substantially more complex helmet display system is disclosed in U.S. Pat. No. 4,361,384 to Bosserman. This system combines a thin film flat panel display and a wafer diode image intensifier tube. A plurality of semi-transparent optical elements including a fiber optic faceplate, direct the image from the flat panel display by reflection from a visor into the eye of an observer where it can be superimposed over the images received from an exterior scene. The observer thus views the exterior scene through both the visor and by way of the flat panel display images. The visor, for reflecting purposes, can be holographic, toric, aspherical or spheric.
U.S. Pat. No. 4,761,056 to Evans et al., describes a display apparatus which includes a combination visor adapted to be attached to the headgear of a user. The visor has a viewing area segment positioned in the line of sight of the user of the headgear. An optical display generates an image and the image is reflected by a parabolic mirror in the optical path from the optical display to the visor viewing area segment. A fold mirror is positioned to receive the image from the parabolic mirror and direct the image to the visor viewing area segment. This apparently minimizes the size and curvature of the visor without affecting transmission of the image.
As is known, the ideal situation for a visor display system would include an infinitesimal image source directly in front of the eye and positioned as close as possible to the focal point of the visor leading to total reflection of an image from the image source to the visor and back to the eye. As the image sources are moved away from the ideal situation, aberrations are introduced into the system which require correction. Various systems correct for such aberrations by means of lenses in front of CRTs for refraction therethrough or as in Evans, a plurality of mirrors for improving the optical transmission and performance.
Various holographic elements which are deposited on visors of display systems have been used to correct for certain aberrations of display systems. However, as with lenses which use refraction to perfect the image, such solutions raise additional aberration problems, especially when utilized in conjunction with color image sources. Utilization of lenses to provide for aberration correction prohibit the various colors of a color image to reach desired destinations at equivalent times and with equivalent intensities. Holographic films on visors are also problematic with color images as the holographic films bend the various color components of the image at different angles.
In addition, the availability of helmet mounted display devices which are of light weight and small size is also a problem. Because the helmet of the display must be worn by an individual with many complex tasks to perform, usually under great stress and with little time in which to accomplish the tasks, the helmet should be as light and compact as possible while still providing superior optical performance. Specifically, bulky and large helmets cause their wearers to tire rapidly, thus shortening the effectiveness to the wearer. Similarly, poor images cause miscalculation as well as hinder the wearer's decision and reaction capabilities. Therefore, there is a need for further reduction in the size and weight of visor display systems. In addition, such a system should provide superior optical performance of not only a monochromatic image but also for color images.